The present invention relates to a semiconductor light-emitting device to be used for, for example, a communication device, a road, rail way, or guide display panel device, an advertisement display device, a mobile telephone, a display backlight, lighting equipment, or the like, and a method of manufacturing the semiconductor light-emitting device.
In recent years, technologies of manufacturing a semiconductor light-emitting diode (referred to as an “LED” hereinafter), which is one of semiconductor light-emitting devices, have rapidly progressed, and in particular, LEDs for primary colors of light have been completed after the blue LED was developed, so that it has become possible to produce light of every wavelength by combinations of LEDs for primary colors of light. As a result of this, the scope of application of LEDs has been rapidly widened, and in particular, in the field of lighting, attention is being given to an LED as a natural-light or white-light source which is an alternative to an electric bulb or fluorescent lamp, with the increase of awareness of environmental and energy issues.
However, current LEDs are inferior in efficiency of conversion of applied energy into light as compared with an electric bulb or fluorescent lamp, and therefore research aimed at developing LEDs having a higher conversion efficiency and higher luminance has been underway.
In the past, the focus of the research and development of a higher luminance LED was on epitaxial growth technologies. However, the intracrystalline illumination efficiency (internal quantum efficiency) has been sufficiently improved by the optimization of the band structure such as a multiquantum well structure, meaning that the technologies have matured. Therefore, the approach to an increased luminance of LEDs is being sifted to the development which centers on process technologies.
Increase in luminance by a process technology means increase in external extraction efficiency, and specifically there are process technologies such as technologies for microfabricating LEDs, and forming reflecting films and transparent electrodes, etc. Among others, some techniques of increasing the luminance by wafer bonding have been established for red and blue LEDs, and high luminance LEDs were invented and have appeared on the market.
Techniques of increasing the luminance by wafer bonding are broadly divided into two types.
One is a technique of attaching an opaque substrate such as a silicon substrate or a germanium substrate to an epitaxial layer directly or through a metallic layer. The other one is a technique of attaching a substrate which is pervious to an emission wavelength, such as a glass substrate, a sapphire substrate, or a GaP substrate, to an epitaxial layer directly or through a bonding layer.
The former allows the attached substrate or the metallic layer to function as a reflecting layer to increase the luminous by reflecting light, which is absorbed by a substrate for epitaxial growth in a conventional LED, to the outside before absorbing the light. The latter extracts light to the outside through a transparent substrate to increase the efficiency of extracting light to the outside.
FIG. 1 is a schematic cross-sectional view of a semiconductor light-emitting device in which an example of the former technique is used. In FIG. 1, the reference numeral 101 denotes a silicon substrate, 102 denotes metal for reflection, 103 denotes a luminous layer, 104 and 105 each denote an electrode, and the reference symbol L denotes emitted light.
FIG. 2 is a schematic cross-sectional view of a semiconductor light-emitting device in which an example of the latter technique is used. In FIG. 2, the reference numeral 201 denotes a transparent substrate, 202 denotes a luminous layer, 203 denotes a window layer, 204 and 205 each denote an electrode, and the reference symbol L denotes emitted light.
In particular, the technique of attaching a transparent substrate to an epitaxial layer does not use reflection, so that light emitted by the luminous layer does not pass through the luminous layer again, thereby being not absorbed by the luminous layer. As a result, it is possible to develop an LED which is capable of extracting the emitted light from substantially the whole surface of the device to the outside and has a higher conversion efficiency (light extraction efficiency).
As conventional techniques of attaching a transparent substrate to an epitaxial layer, techniques of attaching a GaP (gallium phosphide) transparent substrate directly to an AlGaInP (aluminum gallium indium phosphide) semiconductor layer which is a 4-element LED structure part are known (see, for example, JP3230638B2, JP3532953B2, and JP3477481B2).
In the case of a technique of attaching a transparent substrate to a semiconductor layer, an electrode is formed on a non-joint surface of the transparent substrate, while the interface between the metal of the electrode and the transparent substrate which are in ohmic contact with each other is generally an alloy layer. The alloy layer absorbs light which has passed through the transparent substrate, so that the larger the area of the electrode, the more the loss of light increases. Furthermore, when the area of the electrode is reduced to reduce the loss of light, the electrical resistance between the electrode and the transparent substrate increases, so that there arises a problem that the driving voltage of the device increases.
A problem similar to the above problem arises also when an opaque substrate such as a silicon substrate is attached to an LED structure part through metal.
When an opaque substrate is attached to an LED structure part through metal, metal for reflection may be formed on the whole of the joint surface of the opaque substrate. However, heat treatment and the like in the joining process makes the metal for reflection react with the metal for electrical connection so that the metals become an alloy layer reducing the reflection factor or become a light absorbing layer.
Thus, any one of the above attaching techniques has a problem that light is absorbed by the electrode or the metal for reflection, and the reflection effect is thus reduced.